Method for stabilizing the driving state of a utility vehicle combination

ABSTRACT

According to the invention, the driving stability of a utility vehicle combination, which is comprised of a towing vehicle and of a towed vehicle, is monitored ( 3 - 5 ) when the towed vehicle is braked. In the event of an unstable driving state, the vehicle combination is stabilized by effecting a cyclic or clocked reduction of the towed vehicle brake pressure ( 6 ).

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a method of stabilizing the drivingcondition of a utility vehicle combination a towing vehicle and a towedvehicle (trailer).

In the future, utility vehicles will increasingly be equipped with anantilock braking system (ABS), a wheel slip control system (ASR) and anelectronic stability program (ESP) in order to stabilize the vehicle orthe vehicle combination in critical driving situations. By means of ABSand ASR systems, the brake pressures at the individual wheel brakecylinders are individually controlled such that a locking or a spinningof the wheels is avoided.

By means of an ESP system, electronically active brake pressures arecontrolled-in at one or several wheel brake cylinders of the towingvehicle and/or of the trailer in critical, that is, unstable, drivingconditions, particularly when there a danger of swerving, in order tostabilize the vehicle or the vehicle combination. In this case,depending on the driving condition, also longer braking interventionsmay also occur at the trailer or semitrailer. Under unfavorableconditions, particularly when the tires are in poor condition or the ABSoperates defectively, such longer braking interventions may result inthe lateral “swinging-out” of the trailer or semitrailer.

This problem occurs particularly in the case of trailers or semitrailerswhich are braked when cornering. Braking then leads to a loss of thelateral control, which may cause a swinging-out. A “swinging-out” mayoccur during a braking by the driver as well as during an electronicbraking intervention by the ESP.

It is an object of the invention to avoid the swinging-out of thetrailer.

This object is achieved by a method of stabilizing the driving conditionof a vehicle combination including a towing vehicle and a trailer, themethod comprising the acts of a) monitoring whether a brakingintervention is taking place at the trailer; b) sensing one or moredriving condition quantities characterizing the momentary stability ofthe vehicle combination; c) determining whether an unstable drivingcondition is present or imminent; and d) lowering the braking torques atthe trailer wheels when an unstable driving condition is detected instep c) during a braking intervention. Advantageous embodiments andfurther developments of the invention are described and claimed herein.

The basic principle of the invention consists of monitoring the drivingstability of the vehicle combination during the braking of the trailervehicle and stabilizing the vehicle combination in the event of anunstable driving condition by the cyclic or clocked reduction of thewheel braking torques of the trailer or of the trailer brake pressure.

In other words, the wheel braking torques of the trailer or the brakepressure to the trailer or semitrailer are “modulated”. “Modulating”means that, during the operation of the trailer brakes, short phases arerepeatedly “inserted” which have no brake pressure or only a slightbrake pressure. This can take place as a function of the drivingsituation; that is, in the case of an unstable towing vehicle and athereby required ESP intervention at the trailer/semitrailer, the brakepressure of the trailer or semitrailer is controlled in a clockedmanner.

Preferably, a modulation or clocking of the trailer wheel brakingtorques takes place only in really critical driving situations. For thispurpose, the “moving condition” of the vehicle combination has to bemonitored. The moving condition, or the momentary stability of a vehiclecombination is a complicated vector quantity, which can be approximatelydescribed by a plurality of variables.

Particularly, the vehicle speed or the rotational speeds of individualvehicle wheels, the steering angle, the articulation angle between thetrailer and the towing vehicle, the articulation angle velocity, thelateral acceleration, the axle load distribution, etc. arecharacteristic with respect to the stability of a vehicle movement.

For measuring or determining these quantities, various sensors areprovided on the towing vehicle or on the towed vehicle (trailer), suchas rotational wheel speed sensors, yaw rate sensors, etc. For detectingthe “swing-out danger”, sensors can also be provided for measuring ordetermining the articulation angle between the trailer and the towingvehicle.

The clocking or modulating of the trailer wheel braking torques can takeplace during braking by the driver, as well as in the case of an ESPbraking intervention.

The invention is particularly suitable for a semitrailer unit with asemitrailer tractor equipped with an ESP. The semitrailer unit ispreferably equipped with a sensor for measuring the articulation anglebetween the tractor and the semitrailer.

However, as an alternative thereto, the invention is also suitable for asemitrailer unit which is equipped with an ESP but has no articulationangle sensor. In the case of an ESP-controlled braking of thesemitrailer, for example, on a road with a low coefficient of friction,the brake pressure to the semitrailer controlled in by the ESP isreduced to zero at certain time intervals. As explained above, thesemitrailer is thereby stabilized and a swinging-out is avoided.

When no articulation angle sensor is present, the articulation angle canbe estimated on the basis of different available driving conditionquantities and of a specifically defined mathematical-physical vehiclemodel. For this purpose, sensors can be provided which measure the“orientation” of the tractor and of the semitrailer, from which thearticulation angle can be determined. In addition, signals can beanalyzed which are supplied by a navigation or communication systempresent in the vehicle.

On the basis of the defined vehicle-specific mathematical-physicalmodel, the ESP determines a desired value for the articulation anglebetween the towing vehicle and the semitrailer.

When the amount of the determined articulation angle is clearly abovethe desired value, that is, when a significant deviation exists betweenthe desired value and the actual value and the semitrailer is brakedsimultaneously, it is assumed there is a danger that the semitrailer mayswing out.

For stabilizing this unstable driving condition, as a result of the ESP,the brake pressure to the semitrailer is reduced to zero at certain timeintervals. As a result, the wheels of the semitrailer can “run up”again, that is, accelerate, whereby lateral forces are built up and thesemitrailer is stabilized.

When no sensor is available for measuring the articulation angle betweenthe trailer and the towing vehicle, and the articulation angle can alsonot be determined or estimated in another manner, the swinging-outproblem can also be countered “preventively”. This means that theclocked controlling-in of the brake pressure or the repeated “brakingforce reduction” takes place in certain situations, for example, whenthe road is very smooth, specifically without the knowledge of thearticulation angle. It may be provided that in this case only theinformation supplied by the towing vehicle is taken into account.

The signal for reducing the wheel braking torques or the trailer brakepressure can be transmitted, for example, pneumatically, electrically orby radio from the towing vehicle to the trailer vehicle.

In the following, the invention will be explained in greater detail bymeans of an embodiment in connection with the drawing.

BRIEF DESCRIPTION OF THE DRAWING

The single FIG. 1 is a flow chart of the process on which this inventionis based.

DETAILED DESCRIPTION OF THE DRAWING

Beginning with a starting step 1, it is constantly checked in a step 2whether a braking intervention is taking place at the trailer. If nobraking intervention is taking place, step 2 is implemented again.

However, if a braking intervention is taking place at the trailer, the“stability” of the actual driving condition is measured in step 3; forexample, by measuring the articulation angle between the towing vehicleand the trailer vehicle, the rotational wheel speeds, the longitudinaland lateral accelerations, the wheel slips, the axle load distribution,the road inclination, etc.

Diverse “condition vectors”, which characterize critical drivingconditions, are stored in the electronic braking system of the vehicle.Such “critical driving condition vectors” may be generated by individualquantities of the above-mentioned driving condition quantities or bycombinations of the above-mentioned driving condition quantities. Thestored driving conditions vectors are schematically indicated in FIG. 1by means of block 4.

In step 5, the actual driving condition is compared with the definedcritical driving condition vectors. If the comparison shows that thedriving condition is not unstable, a return takes place to step 2; thatis, the braking intervention by the driver or the ESP is not “modified”.

However, if the driving condition is critical, the wheel braking torquesof the trailer or the trailer brake pressure are modulated, that is,clocked, in step 6. The trailer brake pressure or the wheel brakingtorques are therefore briefly lowered. Subsequently, a return to step 2takes place and the entire monitoring routine is implemented again.

1. A method of stabilizing a driving condition of a vehicle combinationincluding a towing vehicle and a towed vehicle, the method comprisingthe acts of: a) monitoring whether a braking intervention is takingplace at the towed vehicle; b) sensing one or more driving conditionquantities characterizing a momentary stability of the vehiclecombination, wherein, using a defined mathematical-physical vehiclemodel, an approximate articulation angle between the towing vehicle andthe towed vehicle is determined and compared with a desired articulationangle, which desired articulation angle is determined as a function of amomentary driving condition and characterizes a stable drivingcondition; c) determining whether an unstable driving condition ispresent or imminent based on the momentary stability by determiningcoefficients of friction which describe an actual or threatenedinstability; and d) modulating braking torques at wheels of the towedvehicle when an unstable driving condition is detected in step c) duringa braking intervention by varying brake pressure of the towed vehicle ina clocked or cyclical manner by repeatedly inserting short phases havingno brake pressure or only a slight brake pressure, wherein the brakingtorques at the towed vehicle wheels are modulated as a function of thedetermined coefficients of friction with respect to their amplitude andtheir time response.
 2. The method according to claim 1, wherein, instep c), a comparison is made between the one or more sensed drivingcondition quantities and one or more assigned driving conditionquantities characterizing an unstable driving condition.
 3. The methodaccording to claim 2, wherein, in step b), using a definedmathematical-physical vehicle model, determining an approximatearticulation angle between the towing vehicle and the towed vehicle, andfurther wherein the determined articulation angle is compared with adesired articulation angle, which desired articulation angle isdetermined as a function of a momentary driving condition andcharacterizes a stable driving condition.
 4. The method according toclaim 1, wherein a brake pressure of the towed vehicle is lowered tozero in step d).
 5. The method according to claim 1, wherein, in thecase of an ESP intervention, the towed vehicle brake pressure islowered.
 6. The method according to claim 1, wherein the towed vehiclebrake pressure is lowered during a braking operation by a driver.
 7. Themethod according to claim 1, wherein, in step a), an intensity of thebraking intervention at the towed vehicle is determined, and furtherwherein the modulating of the braking torques takes place as a functionof the intensity of the braking intervention.
 8. The method according toclaim 1, wherein, in step d), the modulating of the braking torques byusing a pulse-pause ratio and a lowering amplitude is further modulatedby the degree of the actual or threatened instability of the drivingcondition.
 9. The method according to claim 1, wherein, in step d), themodulating is coordinated with other safety, assistance or comfortsystems of the vehicle.
 10. The method according to claim 1, wherein, instep c), an articulation angle occurring between the towing vehicle andthe towed vehicle is precalculated on the basis of a definedmathematical-physical vehicle model.